1. Field of the Invention
The present invention relates to a control method of and control system for an automatic transmission for an automotive vehicle. The automotive vehicle has an engine which is provided with a fuel supply cut-off signal generating means for generating a fuel cut signal as long as the engine speed is higher than a predetermined engine speed after the accelerator pedal has been released to cause the automotive vehicle to begin a coasting operation and also with a fuel supply cut-off means responsive to the fuel supply cut-off signal for suspending the supply of fuel to the engine.
2. Description of the Prior Art
Engine having a fuel supply cut-off have been developed to suspend the supply of fuel (hereinafter called a fuel cut-off) upon release of the accelerator pedal to cause the vehicle to begin a coasting operation at an engine speed higher than a predetermined engine speed (for example, a 1250 rpm during normal operation) for prevention of exhaust gas emission and for enhancement of fuel economy because under this vehicle operating condition, combustion within the engine (the power of the engine) is not required. The arrangement is such that if, during the coasting operation, the engine speed drops to a certain predetermined engine speed (for example, 1000 rpm during normal operation) below which the engine could not be restarted even if the supply of fuel is resumed, the fuel supply cut-off is released to resume the supply of fuel so as to cause the engine to operate again. Subsequently, if the vehicle decends a long slope with an intermediate gear ratio being selected, the engine speed increases even during coasting operation and if the engine speed value increases to a certain engine speed (for example, 2200 rpm during normal operation), fuel cut-off is resumed.
Vehicles which have an automatic transmission and an engine equipped with a fuel supply cut-off means of this kind are capable of running by transmitting the power from said engine to traction wheels via the automatic transmission. Since many automatic transmissions include a torque converter in its power transmission system, it can not establish non-slip drive connection between the engine and traction wheels when the magnitude of driven power, viz., the power transmitted from the traction wheels to the engine, is relatively small during coasting operation so that upon releasing the accelerator pedal to cause the automotive vehicle to begin a coasting operation, a drop in the engine speed is rapid as compared to that encountered in an automotive vehicle having a manual transmission. Thus, a problem exists in that the engine may stall upon depressing a brake pedal for emergency stopping of the automotive vehicle. Hence, it is the common practice that in the case of the automotive vehicle having an automatic transmission, the fuel must be resupplied at a relatively high engine speed value as compared to the automotive vehicle having a manual transmission.
As explained above, the engine speed drops at a fast rate in the automotive vehicle having an automatic transmission during a coasting operation of the vehicle because the torque converter of the automatic transmission can not transmit a torque from the traction wheels to the engine fully. Thus, the time period when the engine speed remains above the predetermined engine speed and the engine runs without the supply of fuel is short.